In a typical kiln plant having two preheaters one of which is heated by means of the kiln exit gases and the other one of which is heated by means of spent cooling air, a kiln and a cooler are coupled after the kiln for cooling the product treated in the kiln. The raw material treated in the preheater and heated by the kiln exit gases may either be passed directly from the preheater to the kiln or via a precalciner coupled to the preheater and precalcining the raw materials after their preheating. The preheaters, of a type which is known to those skilled in the art, for example shaft preheaters or cyclone preheaters, together with their outlet and inlet pipes and fans are herein respectively referred to as "the kiln string", covering the preheater heated by the kiln exit gases, and "the cooling air string", covering the preheater heated by spent cooling air from the cooler.
The cooler may be a grate cooler such as is commonly used in such plants, especially within the cement industry. This type of cooler has, unfortunately, the disadvantage that it requires an amount of cooling air substantially exceeding the amount of combustion air needed for and used in the combustion process in the kiln and possibly also the calciner. Thus, there is excess air from the cooler which represents a considerable heat loss and also a serious problem of dedusting and consequential pollution.